This invention relates generally to apparatus for well logging and more specifically to apparatus for maintaining a plurality of borehole-contacting pads in general contact with the sidewalls of a borehole and for functionally determining the diameter of such borehole.
In many types of well logging operations, during traversal of the borehole, it is desirable to maintain some portion of a logging instrument such as one or more contact members in generally constant contact with the sidewalls of the borehole, not withstanding anomalies or washouts in the surface of the sidewalls of such borehole. In certain logging operations these contact members may be contact pads which may also contain apparatus facilitating the performance of the logging operation. It is also desirable in many of these logging operations to measure the diameter of the borehole throughout the logging operation, including the borehole diameter as the aforementioned sidewall anomolies or washouts occur. It is further desirable in some of these same logging operations to maintain an attached string of additional logging instruments positioned against the general contour of the borehole sidewall.
The prior art has attempted to achieve one or more of these desired goals by utilizing a variety of logging instrument designs. One such design is the "fixed pad" design of a logging device, such as for a logging device often used for density logging operations, wherein a borehole-contacting logging pad containing logging apparatus is pinned, or in some other way fixed, in a slightly laterally extended position relative to the body of the logging instrument prior to insertion of the logging instrument into the well. Diametrically opposing the logging pad, relative to the body member of the logging instrument, is a widely extendable, generally radially moving, decentralizing arm and an attached pressure pad which serve to keep the body of the logging instrument generally decentralized relative to the longitudinal axis of the borehole and to keep the logging pad held against the borehole sidewall. With this fixed pad design the logging pad is subject to considerable wear as it is in generally constant contact with the borehole sidewall as the instrument travels both into and out of the well. Additionally, because the pad is fixed in place it is not capable of maintaining contact with the borehole sidewall throughout a washout in the sidewall if such washout is of a dimension greater than the fixed extension of the pad. Similarly, where there is no washout in the borehole, the pad will act to hold the body member of the logging instrument and, in turn, any other instruments attached to the body member, out of contact with the borehole sidewall.
One attempt to avoid some of the disadvantages of the fixed pad design has been to utilize an alternative design incorporating a hydro-mechanical mechanism to laterally extend the logging pad, as well as the pressure pad after the logging instrument has been placed in the borehole. In addition to disadvantages due to the complexity of a hydraulic system, in design configurations wherein the movement of the logging pad is restricted to a slight lateral extension similar to that utilized with the fixed pad, there is again a limitation on the extent to which borehole sidewall contact may be simultaneously maintained by both the logging pad and the instrument. A hydro-mechanical mechanism has also been utilized in configurations designed to widely extend both the logging pad and the decentralizing arm in unison away from the logging instrument in an attempt to facilitate a greater range of extension for the logging pad so as to promote the maintaining of the logging pad in constant with the borehole sidewalls. This design, while allowing the logging pad to maintain contact with the borehole sidewall, does not facilitate the positioning of the body member of the instrument and any attached string of logging instruments against the sidewall of the borehole, but rather serves to maintain the body member of the instrument generally centralized along the longitudinal axis of the borehole.
The present invention overcomes the deficiencies of the prior art by providing a mechanism by which a plurality of borehole-contacting pads of a logging instrument may freely follow the contours of the sidewalls of a borehole, independent of the relative position of the body member of the logging instrument within the borehole, and, by which an indication may be provided of the diameter of the borehole across such pads.